We have continued our combined ultrastructural and physiological studies of gap junctions between reaggregated Novikoff hepatoma cells. During the past year we have used computer-assisted video analysis to determine the permeability of the junctions to lucifer yellow CH and have begun to correlate permeabilities with cell size/growth state and to compare permeabilities with junctional ionic conductances. In general, the permeabilities are consistent with our previous estimates of junctional size and models for junctional channels. We also have carried out an extensive study of temperature effects on the formation of gap junctions, viewed in freeze-fracture, and of electrical coupling and ionic conductance. We have shown that decreasing temperature decreases formation, as expected, but formation does occur even at 4~C. These results are consistent with our formation model which suggests that junctions develop by a self-assembly process involving migration of junctional precursors within the plane of the membrane. Recently, we have begun attempts to isolate gap junctions from Novikoff cells (in collaboration with Mr. Michael Atkinson) and will soon set up equipment to begin whole-cell patch clamp as a means for detecting formation of the earliest junctional channels. We expect these studies to increase our understanding of the structure and dynamics of the plasma membrane, to give additional insight into the regulation of cell junctions, to provide new tools for testing the biological significance of junctional communication, especially in the control of cell proliferation, and to offer further basis for evaluating the possible importance of altered junction formation in cancer or other pathological states.